The plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria (Xcv) is the causative agent of bacterial spot disease in tomato and pepper, and requires a functional type III secretion system (TTSS) for its pathogenicity. Gram-negative pathogens of both plants and animals use TTSSs for introducing bacterial effector proteins directly into the host cell cytoplasm. The TTSS apparatus is a multi-component complex composed of proteins conserved in all pathogens and proteins unique to each pathogen. There is little information about the identity or function of plant pathogen-specific proteins required for type 11I secretion. The study of these proteins will provide insight into the initial stages of Xcv pathogenesis. The goal of the research in this proposal is to identify plant pathogen-specific components of the Xcv TTSS. Specifically, the TTSS complex will be biochemically isolated and the constituent proteins identified. Xcv-specific proteins will be characterized by determining their sub-cellular localization and their requirement for protein export from the bacteria and/or protein translocation through the host cell plasma membrane. The function of the Xcv HrpF protein, which is required for translocation of effectors into the plant cell, will also be investigated.